1. Field of the Invention
This invention relates generally to semiconductor structure, and more particularly, to structure for suppressing semiconductor chip curvature and reducing chip temperature while improving device speed and reliability.
2. Discussion of the Related Art
FIG. 1 illustrates a typical high-power semiconductor device in the form of a silicon chip or die 12 mounted and secured to a ceramic, for example, alumina, substrate 14. In order to mount and secure the semiconductor die 12 to the substrate 14, the die 12 and substrate 14 are positioned as shown in FIG. 1 and the temperature of these components is raised until the solder balls 16 on the underside of the die 12 and corresponding solder pads on the substrate 14 melt or liquefy sufficiently to form solder connections between the die 12 and substrate 14. Then, the structure is cooled so that the solder connections solidify and the die 12 is secured to the substrate 14.
While the solder is still in liquid form, the substrate 14 and die 12 remain in their original, substantially flat configuration. However, once the solder solidifies, securing the die 12 to the substrate 14, as the assembly is further cooled and both the die 12 and substrate 14 contract, a difference in coefficient of thermal expansion between the die 12 and the substrate 14 will cause the die 12-substrate 14 assembly to bend in the same manner as a bimetal strip.
In a typical prior art system such as described above, with silicon having a coefficient of thermal expansion of 3 ppm/xc2x0 C., and alumina having a coefficient of thermal expansion of 7 ppm/xc2x0 C., as the assembly is cooled from the solidifying temperature of the solder to room temperature, the alumina, for a given change in temperature, contracts more than the silicon, causing the assembly to bend so that the top surface 17 of the die 12 is slightly domed (FIG. 2).
Subsequently, after application of a thermal grease 18 or the like to the exposed surface 17 of the die 12, a beat sink 22 having a flat bottom surface 24 is positioned as shown in FIG. 2. The difference between the thermal expansion of the silicon (3 ppm/xc2x0 C.) and the alumina (7 ppm/xc2x0 C.) is small enough so as to cause only a slight doming effect of the die 12 (with for example the peak being raised 15 xcexcm as compared to an edge of the die 12). Thus, the entire surface 24 of the heat sink 22 can be brought into close proximity with the entire surface 17 of the die 12. With no substantial gaps between the surface 17 and surface 24, good thermal conductivity is provided from the die 12 through the thermal grease 18 to the heat sink 22, as is desired.
However, in modern semiconductor structures wherein an organic substrate 30 is chosen (FIG. 3), the coefficient of thermal expansion thereof (for example, 18 ppm/xc2x0 C.) is substantially greater than for alumina. The difference between the thermal expansion of the silicon (3 ppm/xc2x0 C.) and the organic substrate material (18 ppm/xc2x0 C.) is sufficient to cause substantial bending of the die-substrate assembly as it is cooled. When eutectic solder or lead-free solder is substituted for high lead solder for the balls 16, while the melting point (liquidus) of the eutectic solder or lead free solder is lower than that of lead-based solder, the solidus temperature is nearly the same for all such compositions and only solidus temperature (complete solidification) is relevant to the amount of bending that will be produced as the assembly is cooled. The problem is magnified when eutectic solder is substituted for lead based solder of the balls 16. This bending causes a large doming effect of the die 12 (with the peak being raised for example approximately 50 xcexcm as compared to an edge of the die 12). Then, after application of a thermal grease 18 or the like to the surface 17 of the die 12, when a heat sink 22 is brought into position as shown in FIG. 4, while good thermal contact is made between the center of the die 12 and the heat sink 22, there exist substantial gaps 32 between the die 12 and the heat sink 22 adjacent the edges of the die 12. Thus, near the edges of the die 12, heat from the die 12 is not properly transferred to the heat sink 22. It is to be noted that proper heat dissipation from near the edges of the die 12 is extremely important, as the high power input/outputs of the die 12 are positioned adjacent the edges thereof. Yet these are the areas where the gaps 32 between the die 12 and heat sink 22 are greatest, causing the poorest transfer of heat from the die 12 to the heat sink 22.
Therefore, what is needed is apparatus for providing that heat from a semiconductor die is properly transferred to a heat sink in close association therewith, even with a substantial difference in coefficient of thermal expansion between the die and a substrate on which it is mounted.
In accordance with the present invention, a semiconductor structure includes a substrate and a semiconductor device secured to the substrate. A stabilizing member is secured to the semiconductor device, with the semiconductor device between the stabilizing member and the substrate. The bending stiffness of the substrate is substantially the same as the bending stiffness of the stabilizing member, wherein:
bending stiffness=Et3, with E=Young""s modulus, and t=thickness.
The coefficient of thermal expansion of the substrate is substantially the same as the coefficient of thermal expansion of the stabilizing member.
In another embodiment, a stabilizing member is secured to the substrate, with the substrate between the die and the stabilizing member. The bending stiffness of the die is substantially the same as the bending stiffness of the stabilizing member, wherein bending stiffness is defined as above. In this embodiment, the coefficient of thermal expansion of the die is substantially the same as the coefficient of thermal expansion of the stabilizing member.
The present invention is better understood upon consideration of the detailed description below, in conjunction with the accompanying drawings. As will become readily apparent to those skilled in the art from the following description, there are shown and described embodiments of this invention simply by way of the illustration of the best mode to carry out the invention. As will be realized, the invention is capable of other embodiments and its several details are capable of modifications and various obvious aspects, all without departing from the scope of the invention. Accordingly, the drawings and detailed description will be regarded as illustrative in nature and not as restrictive.